In the field of industrial robotics, the knowledge of the physical characteristics of the robot is important to calculate the exact position of the robot. These characteristics may be defined by a diagram of the robot design, but will vary depending on the assembling process of the robots and the tolerance of its components. In order to refine the knowledge and thereby enhance the robot accuracy, it is necessary to calibrate the model describing it.
Several methods and equipment for the programming of robots have been suggested in the patent literature. In U.S. Pat. No. 6,535,794 is disclosed a three-dimensional coordinate measuring machine utilizing a mechanical measuring arm having known dimensions in order to create an error map of data, which are used to calibrate the robot.
U.S. Pat. No. 8,457,786 discloses a method and apparatus for calibration of an industrial robot system including at least one robot having a robot coordinate system and a positioner having a positioner coordinate system and being adapted to hold and change the orientation of a workpiece by rotating it about a rotational axis. Target points for the robot are programmed with respect to an object coordinate system. The apparatus comprises a robot controller, at least three calibration objects arranged on the positioner, and a calibration tool held by the robot. The positions of the calibration objects are known in the object coordinate system. The robot controller is configured to determine the positions of the calibration objects with respect to the robot coordinate system, to determine the positions of a first and a second of said calibration objects for at least three different angles of the rotational axis of the positioner, to determine the direction of the rotational axis of the positioner in the robot coordinate system based on the determined positions of the first and second calibration objects for the three angles of the axis, and to determine the relation between the first object coordinate system and the positioner coordinate system by performing a best fit between the known and the determined positions of the calibration objects.
In U.S. Pat. No. 7,818,548 is disclosed a method and system to provide improved accuracies in multi-jointed robots through kinematic robot model parameters determination. Multi-jointed robots are calibrated by using the chain rule for differentiation in the Jacobian derivation for variations in calculated poses of reference points of a reference object as a function of variations in robot model parameters. The method also utilizes two such reference objects, and the known distance there between, to establish a length scale, thus avoiding the need to know one link length of the robot. An iterative method is used to find the optimum solution for improved accuracy of the resultant model parameters. Furthermore, determination of the end joint parameters of the robot, including parameters defining the tool attachment mechanism frame, is disclosed, which allows for interchange of tools without subsequent calibration.
In U.S. Pat. No. 7,853,359 is also disclosed a device and method for calibration of a multi-joint industrial robot.
U.S. Pat. No. 6,070,109 pertains to a robot calibration system, which includes a calibration sensor that provides an indication of when a first reference point, which remains fixed relative to a robot base, is a fixed distance from a second reference point, which is located on the robot arm. The robot arm is moved through a plurality of orientations, and each time that the fixed distance between the two reference points is achieved, robot joint position information is determined. The preferred calibration sensor includes a string, which extends between the two reference points and activates a signal generator each time that the string is taut as caused by the orientation of the robot arm. The generated signal indicates that the two reference points are separated by the fixed distance. The determined robot joint positions are then used to determine a calibration factor which varies depending on the needs of a particular situation. Example calibration factors are useful for correcting errors in robot kinematic information, locating the work cell reference frame and locating the tool center point reference frame.
In U.S. Pat. No. 8,989,897 is disclosed a calibration system for a robot and its peripheral, which system includes an emitter attached to the robot or its peripheral. The emitter emits a laser beam and a receiver also mounted to the robot or its peripheral at a point to permit calibration and for receiving the laser beam and to permit calculations to determine the dimension between the emitter and the receiver.
U.S. Pat. No. 4,753,569 pertains to a method and apparatus for calibrating a computer guided robotic apparatus. At least one camera mounted on the robot arm, or fixed at the work station, views a target comprising a pattern of at least three dots fixed to the other of the robot arm or work station, such viewing taking place as the robot arm approaches the work station to carry out the intended operation. The sensing of any deviation relative to a predetermined target pattern is transmitted to the computer to reset the robot's computerized guidance system.
Thus, although there exists a multitude of ways of calibrating a robot, all of them require some kind of additional active measurement equipment, i.e. measurement equipment comprising sensors, and specialized knowledge of how to use said measurement equipment, and furthermore rely on the accuracy by which the physical characteristics of said equipment is known.